Before the advent of ion exchange membranes and thin, catalytically active, dimensionally stable electrodes, most electrochemical cells were rather massive, as compared to the newer cells. Since they were rather massive, many day-to-day operational conditions (which still exist with the newer cells) did not cause problems within the cells. However, recently there has been a revolution in electrochemical cell design, primarily as the result of the use of ion exchange membranes, and catalytically active, dimensionally stable electrodes. These developments have allowed designers to minimize the distance between the electrodes to increase cell operating current densities and increase cell operating pressures, while at the same time conserving energy that would otherwise be wasted as a result of resistance losses caused by the passage of electrical current through the fluids filling the rather large space between the electrodes. Most modern cells have the electrodes pressed against, or at least, very close to, the ion exchange membrane. Such compact designs work very well and are very efficient. However, they are much more prone to operational problems, then were the older, more massive cells, because of the delicate nature of the ion exchange membranes and of the catalytically active, dimensionally stable electrodes. One problem encountered with the newer design of cells is the problem of pressure fluctuations inside the cell itself caused by the removal of gases and liquids from the interior portions of the cell.
Compact electrochemical cells have an anode and a cathode separated by an ion exchange membrane or diaphragm and are used commercially to electrolyze electrolyte solutions to produce a wide variety of chemicals. Many of such cells produce a gas/electrolyte mixture which must be removed from the cell for recycle or for further processing. For example, electrochemical cells with ion exchange membranes are used commercially to electrolyze an aqueous NaCl solution to form a mixture of hydrogen and a sodium hydroxide solution on the cathode side of the cell and a solution of chlorine and spent brine on the anode side of the cell.
If the gaseous product of electrolysis are not removed from the cell soon after they are produced, gas pockets build up within the cell and prevent electrolyte from contacting portons of the electrodes, leading to inefficient operation. This problem becomes more noticeable as current density and electrode area is increased. The absence of electrolyte at the electrode deactivates that portion of the electrode, and thus causes inefficient operation of the cell. The gas pockets also prevent electrolyte from contacting portions of the ion exchange membrane. The absence of electrolyte at that portion of the membrane, causes the membrane to suffer detrimental changes in its physical and chemical properties. These changes are irreversible and cause permanent damage to the membrane.
Another more serious problem is the creation of severe pressure fluctuations within the cell as a result of the improper removal of a gas/liquid mixture from the cell. The gases and liquids tend to separate in the interior of the cell body electrode chamber or in the outlet port and frequently result in the fluid slugging in the outlet line. As the slugs of liquid and gas flow through the outlet line, they cause severe pressure fluctuations in the line. These pressure fluctuations travel back through the liquid in the line and into the electrode chambers of the cell. Pressure fluctuations as high as about 100 centimeters of water have been measured inside the outlet ports and inside the electrode chambers of such cells. These pressure fluctuations cause the membrane to flex which, when coupled with the fact that a portion of the membrane may not be contacted with electrolyte, frequently causes the membrane to crack or break. An ion exchange membrane that is cracked or broken does not serve its intended function, i.e. to transport ions from one electrode chamber to the other electrode chamber, while remaining substantially hydraulically impermeable. It is not practical to patch cracks in the membrane during operation of the cell, nor is it economical to stop operation of the cell to replace the defective membrane.
The present invention provides a dampening device for use in electrochemical cells to remove gases and liquids from the interior portions of a cell while minimizing pressure fluctuations within the cell which result from slug flow and resulting pressure surges created by the improper removal of gases and liquids from the electrode chambers.